Apparatus for preparation of cl2 by electrolysis of hci and polyvalent metal chlorides

ABSTRACT

THE PRESENT INVENTION RELATES TO A DUAL ELECTROLYTE SYSTEM UTILIZING A DIAPHRAGM ELECTROLYTIC CELL. THE ANOLYTE AND CATHOLYTE, CONTAINING AQUEOUS HCL AND A POLYVALENT REDUCIBLE METAL CHLORIDE, ARE PROCESSED AND RECYCLED SEPARATELY. THE SYSTEM OF THE PRESENT INVENTION PRODUCES HIGH PURITY CL2 AT HIGHER THAN CONVENTIONAL CURRENT EFFICIENCIES.

' March 26, 1974 G. GRITZNER ETAL APPARATUS FOR PREPARATION OF C12 BYELECTROLYSS OF HC1 AND POLYVALENT METAL CHLORIDE Original Filed July 24,1969 United States Patent O 3,799,860 APPARATUS FOR PREPARATION F C12 BYELECTROLYSIS OF HCl AND POLYVALENI METAL CHLORIDES Gerhard Gritzner andJames J. Leddy, Midland, Mich., 1:iguors to The Dow Chemical Company,Midland,

c Original application July 24, 1969, Ser. No. 844,402, now Patent No.3,635,804. Divided and this application Jan. 14, 1972, Ser. No. 218,005

Int. Cl. C22d I 02 U.S. Cl. 204-258 18 Claims ABSTRACT OF THE DISCLOSUREThis is a division, of application Ser. No. 844,402, filed July 24,1969, now U.S. 3,635,804.

BACKGROUND OF THE INVENTION One method of producing C12 by electrolysisof HC1 utilizes a polyvalent metal -chloride in the electrolytesolution. For example, U.S. Pat. No. 2,468,766 describes an electrolysismethod which comprises: introducing an electrolyte containing HC1 and apolyvalent metal chloride, e.g., CuClg, into the space between anode andcathode of a non-diaphragm cell, liberating C12 at the anode, reducingthe polyvalent metal chloride at the cathode, withdrawing theelectrolyte through the porous cathode and reoxidizing the polyvalentmetal chloride with air and HC1 for recycle. Such a method does reducethe cell voltage normally necessary for direct electrolysis of HC1.However, current efficiency suiers due to the inherent owcharacteristics of the system which permit back reaction of dissolvedchlorine and the lower valence state metal chloride.

German Pat. 1,277,216 discloses an electrolysis system which attempts tochange the electrolyte ow pattern by use of a diaphragm between theanode and cathode. By maintaining a pressure differential of zero, thediaphragm reduces reaction of the dissolved C12 (anode side) with thelower valence metal chloride (cathode side). Although this does increasethe current eiciency, there is still room for improvement. The Germansystem carries out the reoxidation of the polyvalent metal chlorideinside the cathode compartment of the cell. Since reoxidation is theslowest reaction taking place in the cell, this limits cell eiilciencyand cell size. Furthermore, improved results are only obtained usingoxygen as the oxidizing gas.

It is a principal object of the present invention to provide a methodand system of preparing C12 by electrolysis of HC1 and a polyvalentmetal chloride.

A further object of the present invention is to provide such a methodand system which has high current efficiency.

THE INVENTION The above and other objects and advantages are found iuthe present invention which utilizes separate, dual stream ilow ofelectrolyte containing hydrochloric acid and a reducible polyvalentmetal chloride. The invention employs an electrolytic cell with adiaphragm which divides the cell into anode and cathode compartments.Anolyte and catholyte yare fed into the anode and cathode ICCcompartments. Preferably the cell is operated to achieve essentiallyzero uid ow through the diaphragm.

The anolyte passes through the anode compartment where the hydrochloricacid is electrolyzed to form gaseous C12. The CL2-containing anolyte iswithdrawn from the cell and the gaseous chlorine separated from theremainder of the anolyte. The anolyte can be replenished in hydrochloricacid by absorption of HCl and recycled through the anode compartment.

The catholyte passes through the cathode compartment where polyvalentmetal -chloride is reduced to the lower valence state metal chloride,e.g. CuCl2 reduced to CugClz. This reduced catholyte is removed from thecathode compartment. The metal chloride can be reoxidized to the highervalence state and the reoxidized catholyte recycled through the cathodecompartment.

The dual stream method of the present invention produces high puritychlorine at extremely high current efciency, on the order of indicatinga minimum of back reactionin the cell between the chlorine produced andthe lower valence state metal chloride. Further the reoxidation of thecatholyte takes place outside of the cell which permits use of moreversatile equipment and apparatus. This permits more eicient operationof the system since the reoxidation rate does not limit the cellreaction.

The anolyte and catholyte can contain hydrochloric acid and anypolyvalent metal chloride which exists in at least two oxidation states,e.g. copper chloride, iron chloride and chromium chloride. Copperchloride is preferred. Although concentration ranges can vary quitewidely and the concentrations can differ between catholyte and anolyte,a particular preferred concentration range is from about 0.5 to |about 2Molar CuClz and from about 3 to about 8 Molar HCl. In fact, if diaphragmand process conditions are appropriate, the catholyte can be essentiallypolyvalent metal chloride and/or the anolyte can be essentially HC1.

The reactions taking place in the system of the present invention usingan HCl-CuCl2 electrolyte are as follows:

These reactions add up to an overall reaction of 2HCl-|-1/2O2- Cl2-{H2OPREFERRED EMBODIMENTS The figure is a schematic flow diagramillustrating one embodiment of the dual flow electrolysis system of thepresent invention.

Referring to the figure, electrolytic cell 10, composed of cathode 11,anode 12 and diaphragm 13, is connected to power source 14. Thematerials of construction of the electrodes are those normally employedin electrolytic cells, such as carbon or graphite. Diaphragm materialscan include synthetic materials which retain substantial strength anddimensional stability such as polypropylene and copolymers of ivinylchloride and acrylonitrile, and polyvinyl chloride.

The anolyte cycle comprises feeding anolyte containing hydrochloric acidand a polyvalent metal chloride is fed into the anode compartment A ofthe cell. The HCl in the anolyte is electrolyzed to produce C12 gas. Theanolyte is removed from the cell and the chlorine gas separated in adegassing unit 15. The depleted anolyte passes through heat exchanger 22into an absorption tower 16 where it is contacted wth HCl in gaseous orliquid form, preferably gaseous. HCl is absorbed in sufcient amount toreplenish the HC1 consumed during electrolysis. The replenished anolyteis then recycled through a filter 23 into the cell.

The C12 gas produced by the present method can be further processed bypassing the gas through a condenser 17, which removes a substantialportion of the HC1 in the C12 gas, and drying in chlorine dryer 18, e.g.utilizing H2804. The HCl-containing condensate can be added back to theanolyte.

In the catholyte cycle, the catholyte is fed through the cathodecompartment C where polyvalent metal chloride is reduced to the loweroxidation state. The reduced catholyte is withdrawn from the cell andcarried through heat exchanger 22 to an oxidation tower where it isoxidized, for example by admixing it with an oxidizing atmosphere, e.g.oxygen-containing gas or dilute chlorinecontaining gas, to reoxidize themetal chloride. While the oxidizer 19 is shown in the figure as oneutilizing concurrent flow, it is understood that any tower or tank unitor the like which permits contact of the oxidizing atmosphere with thereduced anolyte is within the scope of the present system. Thereoxidized anolyte is then recycled through a filter 23 into the cathodecompartment.

The exit gases blown out of the oxidizing tower contain some HCl whichcan be recovered by passing the ygases through a condenser 20. TheHCl-containing condensate can be recycled to the oxidizer. A moreelaborate recovery system, which can |be used but is not necessary tothe present invention, is described in U.S. 2,666,024.

The anolyte and catholyte can be cycled for example by means of pumps 21or other conventional means. It is preferable to control temperaturewhich can be done by use of heat exchangers 22. It is also desirable inmany instances to lter the incoming electrolyte streams to remove anysolids. Any type of filter means 23 can be employed.

While the ligure shows a single cell system, it is understood that aseries of cells connected in series could be employed in the presentinvention. For example, the cells could be arranged so that one sideacts as the cathode for one cell and the other side acts as an anode fora second cell.

The following examples serve to further illustrate the presentinvention. The following general procedure and apparatus were used forthe experiments. Unless otherwise indicated the term electrolyte refersto anolyte and catholyte.

A dual ow system similar to the ligure was set up. Catholyte and anolytewere circulated separately by means of pumps. The flow rate could beregulated from to 650 ml./min. The ow of anolyte and catholyte wasmeasured by means of two rotameters. Copper (I) chloride formed on thecathode was reoxidized with oxygen or air in the oxidizer. Ihe latterwas made out of Pyrex glass, having an active volume of about 66 in.3(21" high). The exit gas stream passed through a condenser. Oxygen orair was fed into the oxidizer through a sintered glass disc (mediumsize). 'Ihe absorption of hydrochloric acid took place in a glass tube.The tubing for the connection was either polyethylene or pure gumrubber.

'Ihe electrode used for the cell was machined from a graphite block. Theactive electrode area was 15 in.2, and the electrodes were spaced IAGfrom the diaphragm. All other parts except the electrode area itselfwere painted with Saran type cement in order to prevent electrolysis onthose surfaces. Teflon or Saran type nipples were used for the inletsand outlets of the electrolyte. Two of these electrodes were clampedtogether with a cloth diaphragm fbetween them to form the electrolyticcell. Leaks were minimized by a Silastc lilm along the 1" frame of thecell by painting the same area on the diaphragm with Saran cement and bymelting the edges of the diaphragm. A copper plate to which the copperleads were soldered was bolted to the external side of each graphiteplate by means of two short brass bolts. The complete setup was placedin a temperature controlled hot-box.

Hydrochloric acid concentration was determined by indicator.

Copper (II) concentration was measured in the catholyte by the standardidometric way.

Copper (I) concentration was determined by the following procedure. Asample was quickly added to a ferrie ammonium sulfate solution insulfuric acid. The amount of iron (II) formed was oxidized to iron (III)with 0.1 N ceric sulfate solution using Ferroin as indicator.

Chlorine was absorbed in 600 m1. of a KI solution (400 g./1.) insulfuric acid, diluted to 1000 ml., and aliquots were titrated with 0.1N sodium thiosulfate.

Gas analyses were made with the Orsat Method.

Current efficiency is determined by comparing the actual amount of C12produced with that theoretically producible.

Diaphrarn: Copolymer of vinyl chloride and acrylonitrile Resistancebetween anode and cathode connectors: 33

Current density, amps/in.

Current eeiency, percent Tempera- Volts ture, C.

Example No.

Table I shows the current eiiciencies of the present system at variouscurrent densities and corresponding voltages at two temperatures. Asindicated, extremely high current etliciencies, or above in almost allcases, are achieved even at very low current densities, i.e. 0.33amps/in?, and correspondingly low cell voltages, i.e. less than 1 volt.

TABLE II Conditions:

Composition of electrolyte: 1.46 M CuClz, 5.03 M HC1 Eleetrolyte ow: Seetable below Diaphragm: Polypropylene Resistance: 17 milliohms Temp.:70-70.5 C.

Electro` Current lyte now Current density, rate, eiciency, amps/in 2Volts mlJmin. percent Table l1 shows two aspects of the present system.First, high current eiciencies can be achieved over a wide range ofelectrolyte ow rates (Examples 11-15 and 16- 19). These examples showedlittle if any decrease in current efficiency when the flow rate isdecreased. Second, current eiiciency can be increased by increasing thecurrent density (Examples 20-24).

TABLE III Conditions:

Composition of electrolyte: 1.40 M CuClz, 6.0 M HC1 Eleotrolyte 110W:510 ml./min.

Diaphragm: Polypropylene Resistance: 17 milliohms Temp.: 70-70.5 C.

Amount Cu+ in Oz flow reox- Cuirent in oxiidized Current density, dizecatholyte, eciency, amps/1n 2 Volts Inl/min. g./l. percent Table IIIreects yet another feature of the present system-the ability to produceC12 at relatively high efficiencies even with incomplete reoxidation ofthe polyvalent metal chloride. Examples 25-28 are similar to those shownbefore lwith a substantial excess of oxidizing gas over that sucient toconvert essentially all of the Cu2Cl2 to CuCl2. However, even where the110W of the oxidizing gas in the oxidizer is reduced, thereby resultingin a significant amount of Cu+ remaining in the recycled catholyte, thepresent system and method continue to show high current etiicienciesespecially at high current densities (Examples 33-36).

A representative chlorine gas analysis for any of the above examples isPercent C12 99.69 CO2 0.24 O2 0.01 N2 0.06

Thus the system and method of the present invention can be employed toproduce high purity C12 from electrolysis of HCl and a polyvalent metalchloride at very high current eiciencies over wide ranges of electrolyteconcentrations, current dens-ities and liow rates.

EXAMPLES 37-41 A larger diaphragm cell, having an active electrode areaof 64 sq. inches, was incorporated into the system of the presentinvention. The cell design was similar to that used in the previousexamples. Air at a ow rate of about 42.5 liters/min. was used in theoxidation tower as the oxidizing gas.

TABLE IV Conditions:

Composition of electrolyte: 1.46 M CuClz, 5.03 M HC1 Electrolyte flow:See table below Diaphragm Polypropylene Resistance: Not measured yg meh.

Table IV demonstrates the high current efficiencies achieved by thepresent system and method using air as the oxidizing gas over a widerange of electrolyte iiow rates. Examples 40 and 41 were taken from acontinuously operated mini-plant setup.

6 EXAMPLES 42-43 TABLE V Conditions Composition of electrolyte: 1.50 MCuCla, 5.5 M HC1 Electrolyte flow: mL/mln.

Diaphragm Polypropylene Resistance: Not measured HC1 concentrate Currentmoles/liter Current Example density, efficiency, o. amps/in.I VoltsAnolyte Catholyte percent What is claimed is:

1. A system for the production of chlorine by electrolysis whichcomprises:

(a) at least one electrolytic cell containing an anode, a cathode, adiaphragm therebetween which separates the cell into anode and cathodecompartments and connecting means to a power source;

(b) means connected to said cell for feeding anolyte and catholyte intothe anode and cathode compartments respectively of the cell, saidanolyte and catholyte containing aqueous HC1 and a polyvalent metalchloride; wherein C12 gas is produced at the anode and polyvalent metalchloride is reduced from a higher oxidation state to a lower oxidationstate at the cathode;

(c) means connected to said cell for removing the anolyte and catholytefrom the cell;

(d) means connected to said anolyte removal means for removing the C12gas from the anolyte;

(e) means connected to said C12 removal means for absorbing additionalHC1 into the anolyte for recycle to the anode compartment of the cell;and

(f) means connected to said catholyte removal means for oxidizing thereduced catholyte to reoxidize polyvalent metal chloride for recycle tothe cathode compartment.

2. The system of claim 1 including a series of electrolytic cellsconnected in series.

3. The system of claim 1 wherein said means to reoxidize the polyvalentmetal chloride is spaced apart from the cathode compartment.

4. A system for the production of chlorine by electrolysis whichcomprises:

at least one electrolytic cell comprising an anode adapted to produceC12 gas positioned in an anode compartment, a cathode adapted to reducea polyvalent metal chloride from a higher oxidation state to a loweroxidation state positioned in a cathode compartment, a diaphragm adaptedto separate the anode compartment from the cathode compartment, and aconnecting means to a power source;

a feed means connected to said cell and adapted to provide an anolyte tothe anode compartment and a catholyte to the cathode compartment;

an electrolyte removal means connected to said cell and adapted toremove the anolyte and the catholyte from said cell;

a -gas removal means adapted to remove the C12 gas from the anolyte;

an absorber means adapted to absorb HC1 into the anolyte for recycle tothe anode compartment and spaced apart from said gas removal means andthe anode compartment;

an oxidizing means adapted to reoxidize the reduced polyvalent metalchloride for recycle to the cathode compartment connected to and spacedapart from the cathode compartment.

5. The system of claim 4 including a heat exchanger adapted to controlthe temperature of the anolyte.

6. The system of claim 4 including a heat exchanger adapted to controlthe temperature of the catholyte.

7. The system of claim 4 including lter means adapted to remove solidsfrom the anolyte and the catholyte.

8. The system of claim 7 including two heat exchangers adapted tocontrol the temperature of the anolyte and the catholyte, and acondenser adapted to remove HC1 from exhaust gases from said oxidizingmeans and to recycle HCl condensate to said oxidizing means.

9. The system of claim 8 including a condenser adapted to remove HC1from the C12 gas.

10. The system of claim 9 including a series of electrolytc cellsconnected in series.

11. The system of claim 10 wherein the anode, cathode and diaphragm arespaced apart from each other and the system is adapted to achieve acurrent eiciency of at least about 90 percent at a current density of upto about one ampere per square inch where the polyvalent metal chlorideis completely reoxidized.

12. The system of claim 10 wherein the anode and cathode are spaced upto about %2 inch from the diaphragm.

13. The system of claim 4 including a condenser adapted to remove HC1from exhaust gases from said oxidizing means and to recycle HC1condensate to said oxidizing means.

14. The system of claim 4 wherein the anode and cathode are spaced up toabout %2 inch from the diaphragm.

15. The system of claim 4 including a series of electrolytic cellsconnected in series.

16. The system of claim 4 wherein the anode and cathode are spaced apartfrom the diaphragm.

\1-7. The system of claim 4 wherein the anode, cathode and diaphragm arespaced apart from each other and the system is adapted to achieve acurrent efliciency of at least about percent at a current density of upto about one ampere per square inch where the polyvalent metal chlorideis completely reoxidezed.

18. The system of claim 17 including a series of electrolytic cellsconnected in series.

References Cited UNITED STATES PATENTS 3,481,847 12/1969 Hine et al.204-128 2,468,766 5/1949 Low 204-128 JOHN H. MACK, Primary Examiner R.L. ANDREWS, Assistant Examiner U.S. Cl. X.R. 204--257 '12225250 UNITEDSTATES PATENT OEETQE CERTIFICATE 0F CORRECTION Patent No. 3,799,860Dated Merenzs, 1974 lnventcnf) Gerhard Gritzner and James J. Leddy It iscertified that error appears in the above-identified patent;`v and thatsaid Letters Patent are hereby corrected as shown below:

j co1umn'4, Table 1, "minieme (7oc-)" should be E inserted i'n line 24after "33": v Column 6; Table V, line 14, after measured" Temp: 72-.73Cshould beinserted; a Column 6, linev l72, after "compartmentff---covnnected ,tomshould bek inserted. v

signed end sealed this 3rd dey of December 1976.y

' (SEAL) l Attest:

MecoY M. GIBSON JR. c:. MARSHALL DANN Commissioner of Patents f A.

Attestng Officer'`

